[time-nuts] Low temperature coefficient capacitors for DMTD

[Bruce Griffiths]

Rick Karlquist wrote:
> Corby Dawson wrote:
>    
>> Bert Kehren and I are working on a low cost DMTD system.
>>
>> We are cloning the NBS 106D design with modern components.
>>
>> You might want to check out PPS dialectric capacitors. (Polyphenylene
>> sulfide)
>>
>> Readily available in many values including large ones.
>>
>> For stability from –55° to +85°C, PPS is virtually flat over this
>> temperature range with a ppm/°C of only 7.
>>      
> That's a change of almost 1000 ppm over the stated temperature
> range.  NP0/C0G is rated at +/- 30 ppm.
>
> Rick Karlquist N6RK
>
>
>    
Actually 30ppm/C is the tolerance on the NP0/C0G tempco.
However the tempco varies with temperature and is typically zero (at 
least for one manufacturer) at around 20C.
However the zero tempco temperature will probably vary from batch to batch.

According to the datasheet Kemet PPS SMT capacitors vary by 2% over the 
-55C to 125C range.
The variation with temperature is nonlinear above 100C.
See attached plot for details.
Second plot shows temperature variation of Wima PPS film caps.
Third plot shows the tolerance range for the temperature variation of 
Siemens C0G/NP0 caps.

Achieving a 7ppm/C tempco for PPS film would appear to be fortuitous and 
perhaps not that reproducible.

Bruce
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